1. Field of the Invention
This invention relates to an optical recording medium having a phase-change recording layer.
2. Description of the Related Art
In recent years, attention has been drawn to optical recording media on which high-density recording as well as additional writing and rewriting (overwriting) of recording information can be performed. The additionally writable and rewritable optical recording media include a phase-change optical recording medium on which information (recording information) is recorded by changing a crystalline state of a recording layer thereof by irradiation of a laser beam thereto, and from which the recorded information is reproduced by detecting the difference of reflectivity caused by the changes in the crystalline state. This phase-change optical recording medium captures attention particularly with the capability of rewriting by modulation of the intensity of a single laser beam, and the capability of recording and reproducing information using an optical system having a simpler construction than an optical system for a magneto-optical recording medium.
In general, to record information on a rewritable phase-change optical recording medium, first, a whole recording layer is initialized to a crystalline state, and then a laser beam having a power (recording power) high enough to heat the recording layer to a temperature above a melting point is irradiated onto the phase-change optical recording medium. At this time, the recording layer of portions of the recording medium onto which the laser beam having the recording power is irradiated is melted, and then rapidly cooled, whereby amorphous recording marks are formed. On the other hand, to erase the recording marks from the rewritable phase-change optical recording medium, a laser beam having a power (erasing power) which can heat the recording layer to a temperature above a crystallization temperature is irradiated to the phase-change optical recording medium. At this time, the recording layer of portions of the recording medium onto which the laser beam having the erasing power is irradiated is heated to the temperature above the crystallization temperature, and then slowly cooled, whereby the recording marks (amorphous portions) are returned to the crystalline state (i.e. erased). Thus, in the rewritable phase-change optical recording medium, it is possible to perform rewriting by modulating the intensity of a single optical beam.
In a phase-change optical recording medium of the above type, as disclosed e.g. in Japanese Laid-Open Patent Publications (Kokai) No. 10-326436 and No. 8-224961, a chalcogenide compound, such as a Ge-Sb-Te-based material or an Ag-In-Sb-Te-based material, which contains Sb as a main component and chalcogen, such as Te or Se, as another main component, is generally used to form a phase-change recording layer, due to a large difference in reflectance between the crystalline state and the amorphous state and relatively high stability of the amorphous state. Today, as described above, it is taken for granted by those skilled in the art that a phase-change recording layer should contain chalcogen. Further, it is also known that various elements including Mn are added as additive elements to the Ge-Sb-Te-based material or the Ag-In-Sb-Te-based material, as disclosed in the above-mentioned publications (Japanese Laid-Open Patent Publication (Kokai) No. 10-326436, etc.). These additive elements are used for effects, such as increasing the crystallization speed(speed of crystal transformation) and enhancing stability against aging. However, too much addition of such additive elements degrades other characteristics or properties of the recording layer. To avoid this, in both of the above-mentioned publications, there is a description limiting the amount of additive elements to 20 atomic % or less.
However, through evaluation of various phase-change optical recording media prepared by changing other component elements than Sb contained in a recording layer as a main component and/or the composition ratio of elements composing a recording layer, the present inventor found that by making the content of Mn still higher, it is possible to form a recording layer having recording characteristics (including rewriting speed and thermal stability) as excellent as or more excellent than those of conventional phase-change recording layers.